Polyimide grinding wheels containing diamonds having a barbed metal coating

ABSTRACT

GRINDING WHEELS BONDED WITH AN AROMATIC POLYIMIDE WHEREIN THE ABRASIVE IS FORMED FROM SYNTHETIC DIAMONDS HAVING A METAL COATING HAVING BARBS FROM 0.3 TO 15 MICRONS IN HEIGHT AND WIDTH OF METAL PROJECTING FROM THE SURFACE.

United States Patent 3,585,013 POLYIMIDE GRINDING WHEELS CONTAINING DIAMONDS HAVING A BARBED METAL COATING Dieter Klaus Bruschek, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Mar. 26, 1969, Ser. No. 810,826 Int. Cl. B2411 3/10; C08g 51/12 U.S. Cl. 51-295 8 Claims ABSTRACT OF THE DISCLOSURE Grinding wheels bonded with an aromatic polyimide wherein the abrasive is formed from synthetic diamonds having a metal coating having barbs from 0.3 to 15 microns in height and width of metal projecting from the surface.

' BACKGROUND OF THE INVENTION Abrasive wheels containing diamonds in a polyimide binder have met with wide acceptance in industry, particularly for wet grinding cemented carbide. These polyimide grinding wheels have had significant advantage over the corresponding phenolic grinding wheels. Phenolic resin grinding wheels may be improved by using diamonds which have a smooth coating of a metal such as copper or nickel, and generally exhibit an improvement in grinding ratio of 80-100% as compared with a similar grinding wheel which contains uncoated diamonds. However, when these same smooth metal coated diamonds are incorporated into polyimide resin bonded grinding wheels only a modest improvement in grinding ratio of 10 to 20% is obtained.

SUMMARY OF THE INVENTION It has now been found that certain metal coated diamonds, when used in conjunction with a polyimide binder, provide compositions with greatly improved grinding characteristics. These diamonds have coatings which are not smooth, as shown in a scanning electron microscope as is the case With conventional metal clad diamonds, but instead have barb-like projections. When using such diamonds in polyimide grinding wheels, improvements in grinding ratio as great as 100 percent have been observed over the corresponding Wheels using uncoated diamonds. This result is surprising since the diamonds with the smooth metal coatings did not improve polyimide bonded wheels significantly. Moreover, the diamonds with barbed metal coatings did not perform in the phenolic bonded wheels to give as great an improvement in grinding ratio as is obtained in the polyimide bonded wheels. Thus, it appears that the barbed metal coatings on the diamonds are selectively beneficial in polyimide grinding compositions.

The degree to which diamonds with barbed metal coatings can improve grinding characteristics in polyimide bonded wheels is reflected by the quality of the base, uncoated diamonds. H. B. Dyer and A. R. ,Roy 1 indicate that four characteristics are considered in preparing a commercial synthetic diamond grit: (a) surface; (b) shape; (c) strength; (d) uniformity. Surface is defined as being of three types in which the diamonds have (1) flat smooth faces on single crystals, (2) nonflat smooth faces with re-entrant angles on single crystals, and (3) surfaces indicative of polycrystalline aggregates. Shape is defined as A New Synthetic Diamond Grit and Its Application to Heavier Carbide Grinding Operations in Diamond Abrasives and Tools, .1. Bur-ls, editor, Pergamon Press, Ltd, London, 1964, p. Ind.

3,585,013 Patented June 15, 1971 blocky, intermediate and plate-like. The strength of the diamonds is dependent upon both shape and surface with the strongest being diamonds of intermediate shape with re-entrant surfaces and the weakest being blocky with smooth surfaces. Uniformity in the diamond particles in a commercial grit, especially with respect to strength, is preferred to nonuniformity with particles having an overall average value for the desired characteristic.

Diamonds whose combined characteristics generally result in the best performance in a polyimide grinding wheel are further described in US. Pat. No. 3,385,684 issued May 28, 1968 to Roger C. Voter.

Diamonds having a Tyler screen size of from 50 to 200 generally are best for the present purpose.

The metal coating can readily be applied to the diamonds by cleaning the diamonds such as with aqua regia. When sufiicient cleaning has been effected, the diamonds are placed in a slowly rotating barrel containing a metal plating solution. The barrel is rotated and a suitable Voltage potential is applied to cause electrodeposition of the metal over the surface of the diamond crystals. The metal should be applied to give a 45 to percent increase in weight to the uncoated diamonds which results in an average increase in a linear dimension of the diamonds of from 10 to 15 percent.

Due to the nature of electrolytic deposition the resultant metal coating is barbed in character. The barbs vary from 0.3 to 15 microns in height and width at the base and due to their somewhat complex pyramidal t0 conical nature cover most of the surface of the diamond. The preferred metals for forming the coating are nickel, copper, tantalum, niobium and cobalt. A further description of metal coated diamonds may be found in Belgian Pat. No. 686,805.

The polyimides used herein are powders of the aromatic polyimides. Generally, these aromatic polyimides have the formula I] H H o o o where R, R and R are organic radicals selected from the group consisting of aromatic, aromatic heterocyclic, bridged aromatic and substituted groups thereof.

Preferred Rs include as @Q -Q wherein the bonds to the carbonyl groups shown are arranged in pairs with the carbonyl groups of each pair on adjacent or peri carbon atoms. Preferred R s include Q @mQw-Q- and When pyromellitic dianhydride is used at least 14 mole percent of the diamines used should contain two aromatic rings. Preferred R s include wherein the bonds to the carbonyl groups are arranged in pairs on adjacent or peri carbon atoms.

R is selected from the group consisting of carbon 1n an alkylene group or perfluorinated alkylene group of from l-3 carbon atoms, oxygen, silicon in Si(R where R is lower alkyl or aryl and R is R or hydrogen. The polyimide powder should have a surface area of at least 0.1 square meter/ gram and preferably greater than 2 square meters/gram, as measured by adsorption of nitrogen from a gas stream of nitrogen and helium at liquid nitrogen temperature, using the technique described by F. M. Nelsen and F. T. Eggertson (Anal. Chem. 30, 1387 [1958]). Sample weights are in the order of 0.1- 3.5 g. The thermal conductivity detector is maintained at 40 C. and the flow rate is approximately 50 mL/min. The gas mixture used is parts by weight nitrogen and 90 parts by by weight helium.

The polyimide should have an inherent viscosity of at least 0.1 and preferably from 0.3 to 5, as measured at 35 C. from a 0.5 percent by weight solution in 96% sulfuric acid. If the polyimide is not soluble to the extent of 0.5 percent in 96% sulfuric acid at 35 C. and a strong article can be coalesced therefrom it is assumed to have an inherent viscosity of greater than 0.1.

The molded abrasive composition generally contains from 6 to 35 volume percent diamonds, from 3 to 17 /2 volume percent metal coating on the diamonds, and from 47 to 91 volume percent aromatic polyimide. Optionally, part of the resin phase can be replaced with up to 50 volume percent as based on the resin phase of another abrasive such as silicon carbide, or aluminum oxide, or a filler such as glass, or a metal in the form of metal powder or metal fibers.

Generally, the diamond containing composition is made up as a rim with the diamonds embedded in a coalesced polyimide matrix which rim is then mounted on a core. The core preferably is aluminum, but other materials such as aluminum filled phenol-formaldehyde resin may be used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preparation and testing of grinding wheels resin in the case of 75 concentration of diamond wheel 7 (a concentration means 25 volume percent or 72' carats of diamonds per cubic inch). In either case, the polyimide is prepared from 4,4-oxidianiline and pyromellitic dianhydride by the technique set forth in US. Pat. No. 3,249,588 issued May 3, 1966 to Walter George Gall, and having a surface area of over 50 equare meters/ gram. 18.62 grams of this composition is compacted at about 4,000 p.s.i. into the cavity formed between a 6-inch diameter circular mold and a concentric 5% inch diameter core. The mold and its contents are then heated to 300 C. and the abrasive composition is further compacted at 50,000 p.s.i. while being maintained at 300 C. to make a preform. The resultant preform and mold is then removed from the press and heat-treated for 12 hours in a nitrogen atmosphere at 300 C. to complete imidization of the resin. The preform rim is then free sintered by heating at 450 C. for 25 minutes in a nitrogen atmosphere. The rim thus formed is fitted over an aluminum core coated C-7 epoxy adhesive made by Armstrong Products Company, and the adhesive is cured. The wheels are 6-inch diameter by inch wide DlAl peripheral wheels.

(B) Grinding wheels with phenolic binders.The diamond-containing composition is prepared by admixing 25 volume percent of diamonds and 75 volume percent of metal coating and a mixture of grams of 400 grit silicon carbide and 42 grams of BRP 5727 medium flow novolac type phenol-formaldehyde resin useful as a wheel binder as made by Union Carbide Corporation. About 34.5 grams of this composition are placed in the cavity formed between a 6-inch diameter circular mold and a concentric 5% inch diameter core and the mold is placed in a press having platens heated to 177 C. A pressure of 3,000 p.s.i. is applied to the composition for 30 minutes during which time the composition is maintained at 177 C. The mold is removed from the press and the rim given a cure in air using the following cycle:

1 hour at 121 C.

2 hours at 177 C.

12 hours at C.

2 hours cooling to room temperature The finished rim is glued onto an aluminum core using an epoxy adhesive and the adhesive cured, to form a 6-inch diameter, inch wide DlAl wheel.

The test results on this wheel are reported as Example 8 in Table I.

The wheels are tested by mounting on a Gallmeyer and Livingston No. 28 grinder and, after truing and running-in on carbide, are used to surface grind a work piece of Walmet WA-S cemented carbide (C-5 grade) 2 inches by 9 inches or 4.2 inches by 11.2 inches under the following conditions:

Wheel speed: 6280 s.f.p.m. (4000 rpm.)

Table speed: 50 f.p.m.

Downfeed: 0.001 in./reversal Crossfeed: 0.050 in./pass Cooling: flood conditions with 2.7 percent aqueous International 218X The grinding ratio, which is the ratio of the volume of carbide removed to the volume of wheel worn, is determined by averaging results from at least three individual grinding tests, removing about 1.2-1.4 cubic inches of carbide per test.

In Table I the phenolic wheels reported as Examples 6 and 7 are high quality commercial wheels. In Table I under the heading Diamond:

a=uncoated diamonds (#1);

b=the same diamonds as in a but with a smooth coating of nickel averaging in thickness approximately 1015% of the average linear dimension of the diamonds;

c=the diamonds of a but with a barbed coating of nickel averaging in thickness 1015% of the average linear dimension of the diamonds, which barbs are approximately 0.3 to 8 microns in diameter at the base and 0.3 to 8 microns in height with several smaller protrusions on each barb;

d=uncoated diamonds (#2); and

e=the d diamonds but with a barbed coating of nickel averaging in thickness -15% of the average linear dimension of the diamonds, which barbs are approximately to microns in diameter at the base with some small protrusions from each large barb.

TABLE I Grit Concen- Grinding Example Binder Diamond size tration ratio As can be seen from the results reported in Table I, the grinding wheel of the present invention as reported in Example 3 gives a much higher grinding ratio than the smooth coated diamond wheel of Example 2 which in turn gives only a small improvement over the uncoated diamond wheel of Example 1. This Example 3 also shows a substantial improvement over Example 5 in which diamonds having a different coating are used. The same relationship holds true in the 75 concentration wheels illustrated in Examples 9-11. This is in contrast to the results obtained with the conventional phenolic bonded wheels wherein the smooth coated diamonds have proven superior to the uncoated diamonds, but no improvement over the smooth coated diamonds is observed with barb coated diamonds.

I claim:

1. An abrasive composition comprising from 6 to 35 volume percent of '50 to 200 Tyler grit size synthetic diamonds, which diamonds are coated with a barbed metal coating which averages in thickness from 10 to 15 percent of the average linear dimension of the diamonds, and which barbs are from 0.3 to 15 microns in height and from 0.3 to 15 microns in width at the base, which metal coating comprises from 3 to 17 /2 volume percent of said abrasive composition, and from 47 to 91 volume percent a phase, at least 50 volume percent of which is a coalesced aromatic polyimide.

2. The composition of claim 1 wherein the composition is in the form of a grinding wheel.

3. The composition of claim 2 wherein the aromatic polyimide is prepared from 4,4'-oxydianiline and pyromellitic dianhydride.

4. The composition of claim 3 wherein said barbed metal coating is nickel.

5. The composition of claim metal coating is copper.

6. The composition of claim 3 wherein said barbed metal coating is tantalum.

7. The composition of claim metal coating is niobium.

8. The composition of claim metal coating is cobalt.

3 wherein said barbed 3 wherein said barbed 3 wherein said barbed References Cited UNITED STATES PATENTS 299,055 5/1884 Collins 51-295 3,295,940 1/1967 Gerow 51298 3,471,276 10/1969 Bragaw 51-298 FOREIGN PATENTS 1,142,688 9/1957 France 5l-295 DONALD J. ARNOLD, Primary Examiner US. Cl X.R. 51-498, 309 

